In an effort to identify symbionin as a sensor kinase of the two-component pathway in the aphid endosymbint, I searched for possible response regulators that are capable of assepting the high-energy phosphoryl group from symbionin. When the endosymbiotic proteins were separated by SDS-PAGE,fixed on a PVDF membrane, and then incubated with symbionin-^<32>P,the radioactivety was found on eight species of polypeptides, indicating that the phosphoryl group was transferred from symbionin to these polypeptides. Since it is possible that these eight proteins function as response regulators in the aphid endosymbiont, we started the study on the characterization of these proteins.Amino acid sequincing and immunochemical analysis of 66,63,42, and 20-kDa proteins revealed that are homologues of B.aphidicola DnaK,B.aphidicola symbionin, E.coli OmpF,and P.milliaris Histone Hl, respectively. Immunohistochemical analysis using electron microscope and subcellular fractionation study demonstrated that OmpF-homologous 42-kDa protein is localized mainly in the outer membrane fraction, while symbionin is only in the cytoplasmic fraction of endosymbiont. It was also shown that the phosphotransfer between symbionin and isolated outer membrane proceeds in a time-dependent fashion. Based on these findings, I peoposed a model for the signal transduction system mediated by the two-component pathway consisting of a stress protein "symbionin", and an outer membrane protein "OmpF".